White "hobby" enamel, three coats with a small Testors hobby brush. Picture shows just one coat. Cleaned the surface first with 91% rubbing alcohol. Worked well as a test - if I fail, I'll order a new reflector.
Update - I didn't like the white reflector beam. So, I cleaned off the paint, which cleaned off the silver, leaving the black plastic base. Until I order a new reflector, I'm using this 200L NB45 with a glossy black reflector...
The cleaned black base
The enameled white painted over the silver.
The light without a silvered reflector
Ditto